The present invention relates generally to electrostatographic printing apparatus and more particularly to a fusing system for fixing toner material to a support substrate. In particular the present invention relates to a release agent donor member for a toner fixing station in such apparatus.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to in the art as toner. The residual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper with subsequent affixing of the image thereto.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to affect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an abhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. duPont De Nemours under the trademark Teflon. The silicone rubbers which have been used as the surface of the fuser member can be classified into three groups according to the vulcanization method and temperature, i.e., room temperature vulcanization silicone rubber hereinafter referred to as RTV silicone rubber, liquid silicone rubber referred to as LSR rubber, and high temperature vulcanization type silicone rubber referred to as HTV rubber. All these silicone rubbers or elastomers are well known in the art and are commercially available.
In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retained by the heated fuser roller and not penetrate into the substrate surface. This tackified material will stick to the surface of the fusing roller and come in contact with the subsequent sheet of supporting substrate bearing a toner image to be fused. A tackified image which has been partially removed from the first sheet, may transfer to the second sheet in non-image portions of the second sheet. In addition, a portion of the tackified image of the second sheet may also adhere to the heated fuser roller. In this way and with the fusing of subsequent sheets of substrates bearing the toner images, the fuser roller may be thoroughly contaminated. In addition, since the fuser roller continues to rotate when there is no substrate bearing a toner image to be fused there between, toner may be transferred from the fuser roll to the pressure roll. These conditions are referred to in the copying art as "offset". Attempts have been made to control the heat transfer to the toner and thereby control the offset. However, even with the abhesive surfaces provided by the silicone elastomers, this has not been entirely successful.
It has also been proposed to provide toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied on the fuser roll to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
According to prior art techniques, the toner release agents may be applied to the fuser roll by several delivery mechanisms including wicking impregnate webs and by way of a donor roll. One technique in particular has been the use of high temperature vulcanized silicone rubber which are peroxide cured materials. These silicone rubber donor rolls however suffer from difficulty in that they swell by being in contact with the silicone oil which migrates or is absorbed into the silicone rubber. While a small degree of swelling may be acceptable if it is uniform, failure of such rolls has been observed by excessive swelling after a period of operation of about 1 month wherein the donor roll elastomer may actually be twice the original size. Under such circumstances the silicone rubber donor roll may no longer function in providing a uniform layer of release fluid to the fuser roll.
In another embodiment the donor roll may comprise an EPDM (terpolymer elastomer made from ethylene, propylene and diene monomer) core with a thin sleeve of Teflon PFA (E. I. du Pont De Nemours) which is an independent extruded thin sleeve of material which is bonded onto the core. The use of such a sleeve is very expensive and the manufacturing of such a donor roll, is tedious and inefficient, the yield being relatively low since so many of the sleeves are damaged during manufacture. Furthermore, in a fusing assembly such as that illustrated in FIG. 1, which will be described in greater detail hereinafter, such a sleeved donor roll is ineffective in that since the donor roll is driven by frictional engagement with the fuser roll, and the hard Teflon PFA coating has a relatively low coefficient of friction difficulties are presented in providing the necessary driving component.